A prior pressure monitoring device isolator utilizes an intermediary sensing fluid (e.g. liquid) isolated from the fluid (e.g. liquid) in a pipeline, by a resilient pressure transmitting member. Since this isolating sensor does not expose the pressure monitoring device to the fluid being measured, it is quite useful when measuring the pressure of fluids which can be harmful to pressure monitoring devices. U.S. Pat. No. 4,218,926 discloses such an isolating pressure sensor.
Although such a system does isolate the pressure monitoring device from the pipeline fluid, it provides no isolation from the sensing fluid. The sensing fluid in such systems is housed in an annular chamber, one wall of which is a resilient pressure transmitting member. The sensor is attached to the pipeline so that the pipeline fluid pressure is exerted on the resilient member. However, the sensing fluid chamber within the sensor has only a finite volume. Therefore, when a pressure monitoring device is removed from the isolating sensor while the sensing fluid is under pressure of the pipeline fluid, the pressurized sensing fluid is exposed to atmospheric pressure and can spray out from the pressure monitoring device port. Sensing fluid, if lost in this manner, must be replaced, because without sufficient sensing fluid the isolating sensor is inoperable. Such replacement of sensing fluid is a costly inconvenience. These considerations are particularly true when the sensing fluid is noncompressible fluid such as a liquid. Thus, a device which can isolate the pressure monitoring device from the pressure applied to the sensing fluid, for the purpose of changing or maintaining the pressure monitoring device, is desirable.
A prior art device disclosed in U.S. Pat. No. 4,082,002 uses a valve stem movable in a valve bore to disconnect a pressure monitoring device passage from a pipeline. The pressure monitoring device passage is threaded partway to receive a pressure monitoring device. However, in contrast to the present invention, a plug of grease in the pressure monitoring device passage transmits pressure from the pipeline liquid to the pressure monitoring device. A grease passage, stoppered at its outer end adjacent the pressure monitoring device, acutely angles into the outer portion of the pressure monitoring device passage for injection of grease thereinto. The pressure monitoring device passage extends generally tangentially, of and is spaced radially outboard of the central pipeline liquid passage through the device. The valve bore extends from the outer periphery of the device radially inward toward the central pipeline liquid passage. The pressure monitoring device passage meets the valve bore at a 90.degree. angle. The outer ends of the pressure monitoring device passage and valve bore are circumferentially remote, lying almost one fourth of the device perimeter apart. Substantial open space around the device is thus needed for operation of the valve and removal of the pressure monitoring device. This is especially a problem in crowded pipeline environments where access to the pressure monitoring device isolating apparatus is difficult.
Also, the pressure monitoring device passage in this prior art device is, relative to the present invention, of very great volume, since the length of the pressure monitoring device passage is nearly equal to one half the diameter of the entire device, and the pressure monitoring device passage is everywhere at least approximately onehalf the diameter of the threaded pressure monitoring device receiving portion thereof. Thus, even when the valve is closed, removal of the pressure monitoring device allows a relatively large amount of liquid (or grease) to be lost due to spillage.
Also, the threaded valve bore of this prior art device is formed directly in the housing of the device. The valve stem then threads directly into the valve bore. Thus if any thread damage occurs due to use of the valve, the entire housing may require replacement.
Also, to seal against fluid leaks past the valve stem into the outer portion of the valve bore, the valve stem is surrounded by and moves axially through a packing gland which must be held fixedly in its position in the valve bore thereby adding to the complexity of the prior device.
Accordingly, the objects of this invention include provision of:
a pressure monitoring device isolator which avoids the limitations of the prior art discussed above and which is capable of isolating a pressure monitoring device from a supply of pressurized sensing fluid to enable removal and replacement of the pressure monitoring device without significant loss of sensing fluid;
a pressure monitoring device isolator as aforesaid that is conveniently accessible for pressure monitoring device replacement and valve actuation even in crowded pipeline environments;
a pressure monitoring device isolator, as aforesaid, which minimizes the volume of sensing fluid exposed to spillage during pressure monitoring device replacement;
a pressure monitoring device isolator, as aforesaid, having readily replaceable valve elements;
a pressure monitoring device isolator, as aforesaid, that provides a seal for the valve member which is both inexpensive and easily accessible and maintainable;
a pressure monitoring device isolator, as aforesaid, that is of simple, economical construction.
Other objects and purposes of the invention will be apparent to persons acquainted with apparatus of this general type upon reading the following specification and inspecting the accompanying drawings.